1. Field of the Invention
The present invention relates to an active noise control apparatus for a domestic appliance and, more particularly, to a noise canceler which can actively cancel noise which is apt to externally leak from an opening portion of a machine room storing a rotary machine which serves as a driving source for a domestic appliance and is driven by an AC power supply.
2. Description of the Related Art
Recently, various electric appliances have been used at home. These appliances pose a problem in terms of noise in living rooms
For example, compressors which generate noise are integrally assembled in most of domestic electrical refrigerators. Such electrical refrigerators tend to be equipped near the living rooms. For this reason, it is important to eliminate noise leaking from the electrical refrigerator into the living rooms.
In an electrical refrigerator, most of noise components are generated by a compressor and a piping system connected thereto. More specifically, the compressor generates operation noise of a motor, fluid noise caused by a compressed gas, mechanical noise of a compression mechanism portion, and the like. The piping system connected to the compressor vibrates upon reception of the vibration of the compressor, thus generating noise.
For these reasons, an electrical refrigerator employs a so-called machine room storing a compressor as a noise source and a piping system connected thereto, so that the machine room eliminates noise leakage. In addition, the electrical refrigerator employs a rotary compressor which generates relatively low noise, an anti-vibration support mechanism of the compressor is improved, or the shape of the piping system is improved to attenuate vibration in a vibration transmission path. Alternatively, a sound insulating member or a sound shielding member is arranged around the compressor and the piping system to increase a sound insulation amount or to increase a noise transmission loss.
A heat radiation opening for radiating heat generated upon operation of the compressor must be formed in the wall of the machine room. For this reason, noise leaks from this opening. Even when the above-mentioned countermeasures for eliminating noise are taken, a noise level can only be reduced by at most 2 dB (ISO-A characteristics).
Recently, upon application of an acoustic control technique, a tone having an opposite phase to and the same wavelength and amplitude as those of a noise component is artificially generated to actively cancel noise leaking from the opening of the machine room, thereby eliminating noise of an electrical refrigerator. In the active noise control, a noise component from a noise source i converted into an electrical signal by a control tone receiver (e.g., a microphone) arranged at a specific position, and a control tone generator (e.g., a loudspeaker) is operated on the basis of a signal obtained by processing the converted electrical signal by a computer, thereby generating an artificial tone having an opposite phase to and the same wavelength and amplitude as those of a noise component, so that the artificial tone and the noise component as an original tone are interfered with each other, thereby attenuating the original tone.
The active noise control is described in a reference entitled "IEICE (Institute of Electronics, Information and Communication Engineers of Japan) Technical Report Vol. 88, No. 105; Jun. 30, 1988", and will be explained below with reference to FIG. 9.
In FIG. 9, a tone generated by a compressor S as a noise source is represented by X.sub.s, a tone generated by a loudspeaker A as a control tone generator is represented by X.sub.a, a tone received by a microphone M as a control tone receiver is represented by X.sub.m, a tone at a noise cancel objective point O is represented by X.sub.o, and acoustic transfer functions among these tones are respectively represented by G.sub.AM, G.sub.AO, G.sub.SM, and G.sub.SO, the following equation is established in a two-input, two-output system. The acoustic transfer functions G.sub.AM, G.sub.AO, G.sub.SM, and G.sub.SO imply that the former suffixes correspond to the transmission sides, and the latter suffixes correspond to the response sides For example, G.sub.AM represents an acoustic transfer function when an input signal to the loudspeaker A corresponds to an input side and an output signal from the microphone M corresponds to an output side to perform measurement. ##EQU1##
From the above equation, a tone X.sub.a to be generated by the loudspeaker A is given by: EQU X.sub.a =(-G.sub.SO .multidot.X.sub.m +G.sub.SM .multidot.X.sub.o)/(G.sub.SM .multidot.GA.sub.O -G.sub.SO .multidot.G.sub.AM)
In this case, since it is aimed at making an acoustic level at the noise cancel objective point O zero, X.sub.o =0 can be set. As a result, we have: EQU X.sub.1 =X.sub.m .multidot.G.sub.SO /(G.sub.SO .multidot.G.sub.AM `G.sub.SM .multidot.G.sub.AO)
As can be seen from the above equation, in order to make the tone X.sub.o zero at the noise cancel objective point O, a tone X.sub.a obtained by filtering the tone X.sub.m received by the microphone M using a coefficient according to a transfer function G expressed by the following equation need only be generated to theoretically make the acoustic level zero at the control objective point O: EQU G=G.sub.SO /(G.sub.SO .multidot.G.sub.AM -G.sub.SM .multidot.G.sub.AO)
For this purpose, a computer H is arranged.
However, when the active noise control method is employed to reduce noise components of a electrical refrigerator, the following problem remains unsolved. More specifically, in the active noise control method, noise of the compressor S is detected by the microphone M, signal processing is performed on the basis of the detection signal, and an additional tone for canceling noise is generated by the loudspeaker A on the basis of the signal obtained by the signal processing. However, when the microphone M detects noise generated by anything other than the compressor, i.e., noise generated outside the machine room, noise control cannot be performed, and extra noise is undesirably generated by a noise control system.
In order to solve such a drawback, a vibration pickup is attached in place of the microphone to detect a vibration of the compressor, thereby generating an additional tone necessary for noise control. However, with this method, in the electrical refrigerator, when a vibration caused when a door of a vegetable compartment is opened/closed is transmitted to the compressor, and is detected by the vibration pickup, extra noise is also undesirably generated by a noise control system.
The same applies to other domestic appliances other than the electrical refrigerator (e.g., an electric washing machine, a drying machine, a dish washer, air conditioner, and the like).
As described above, even when noise components that are apt to externally leak from an opening portion of a machine room storing a rotary machine driven by an AC power supply are to be actively controlled like in a machine room of a domestic appliance, e g., a refrigerator by the conventional active noise control method, a noise control system undesirably generates extra noise depending on an environmental condition.